As a conventional technique, a gas concentration detection apparatus configured to detect the concentration of oxygen in exhaust gas or the like is known as disclosed in, for example, Patent Literature 1 (Japanese Patent Laid-Open No. 10-177005). The gas concentration detection apparatus according to the conventional technique includes a detection element arranged in an exhaust passage in an engine. The detection element is covered with a double cylinder-like sensor cover provided with a plurality of vent holes. The sensor cover has a function to protect the detection element and a function to stabilize a flow of exhaust gas which comes into contact with the detection element.
Furthermore, as another conventional technique, a PM sensor is known which detects the amount of PM contained in exhaust gas. The PM sensor according to the conventional technique includes two electrodes arranged opposite each other in an exhaust passage. The PM sensor is configured to detect the amount of PM in the exhaust gas utilizing the value of the resistance between the electrodes varying depending on the amount of PM accumulated between the electrodes. Furthermore, the PM sensor includes a sensor cover which covers an area where the electrodes are arranged. The sensor cover has a structure and functions similar to the structure and functions of the sensor cover of the above-described gas concentration detection apparatus.